Why Does Hearing Dampen After Loud Music
This week, researchers in Sydney and in California have identified a chemical factor dampens down hearing, causing the temporary hearing loss that we experience after exposure to loud sounds such as a rock concert. And this can even protect us from the permanent hearing loss that's caused by very long term noise exposure. To find out more, Chris Smith spoke to Gary Housley from the University of New South Wales in Sydney.
Gary - So, what we've discovered is that this receptor which is called a receptor for ATP is activated when sound levels come up in our environment. So, as noise develops, then we found that the hearing organ, the cochlea releases a particular chemical which binds to this receptor and then the receptor triggers a process which is quite remarkable which reflects adaptation or adjusment of hearing sensitivity. This research has identified this one receptor, one molecule as the start of a process that over a period of 20 minutes or more with an increase in background noise turns down the sensitivity of our hearing.
Chris - You said that it kicks in after about 20 minutes. So, how long does the effect last for, once you've got the diminuendo in or reduction in hearing sensitivity? How long do you remain partially deaf for?
Gary - Perhaps, I can put this in the context, the experiment itself. So, having identified a receptor, we wanted to find out what it was doing with regard to the influence of noise on hearing and a drug company in the United States had developed a mouse model where those mice no longer express that receptor.
We were provided with some of those mice and we exposed the mice to 20 minutes an hour or 2 hours of quite loud sound - 85 decibels of noise which is louder than a lawn mower. We were expecting to see a major hearing loss because that's what happens with mice or people when they're exposed to these loud sounds. So, the eureka moment for us was then after all this time in the sound chamber, we tested them out - the hearing of the mice - and there was no change in the hearing sensitivity. It was beautifully acute. So, noise had had no impact on the sensitivity of the mouse.
So, what we then did is of course compared this with a normal mouse where that receptor was present and functioning, and that's when we plotted the time constant for the development of the loss of hearing. And the time constant is roughly a measure of the time it takes to get half that makes more effect, and thats about 20 minutes. So, the next question was, once this hearing desensitisation or adaptation has occured, how long after noises turned off does it take for the hearing to return? We found by testing the hearing various times after we'd turn the noise off that the time constant is about 12 hours for recovery and what that means in practical terms is that after half an hour of loud noise, a normal mouse would take more than 24 hours to regain its baseline level of hearing.
Chris - It takes 20 minutes for that protective effect to have at least half of its effect.
Gary - Yes.
Chris - Sounds can do damage much more accutely than that though. If you go and listen to The Who - the loudest band in history - they're going to start playing chords that are 110 decibels from the minute their hands at the guitar strings. So, I'm surprised the system isn't faster.
Gary - Well, perhaps the good news is that ther are faster response systems for dealing with loud transients and that's part of your feedback control to the hearing organ from the brain. And so, that's called an efferent system and that neural pathway can adjust the hearing sensitivity in a matter of milliseconds per seconds. So, it's a very fast dynamic process that actually enables us to pay attention to particular sounds against a noisy background. But that also adapts very quickly, so it's a dynamic response.
When it comes to recreational noise exposure or perhaps music exposure in the case of The Who, those very intense sounds are worrisome. It's something that as you pointed out, it's not within normal physiological parameters for our hearing organ and there's a high likelihood that will cause damage. I suppose the issues that our research raises is that with this natural ability to adapt to loud sounds, it's likely that some people will have this in a more effective mechanism than others. And so, if people have less dynamic control of their hearing sensitivities through this ATP receptor pathway we've identified then they may well be at risk of hearing loss because we found that with our mice that didn't have this receptor, if we raise the ante as it were and expose them to very loud sounds in excess of 95 or 100 db which is aproaching those sorts of levels you talked about then after some time of exposure, they had far risk permanent hearing loss than the normal mouse that has the fully developed adaptation reflex.
Chris - So, before I go to my next rock concert, should I be popping a pill of something or is this the ultimate aspiration? We can give people pharmacological ear defenders then.
Gary - So, the neat thing about these sorts of elements is perhaps the potential for prophylactic treatment. One of the challenges from noise-enduced hearing loss has been that the safe levels of sound exposure are defined but people get caught out or they make a voluntary decision that they're going to ignore those kinds of prudent guidelines.
So, if indeed there is that therapeutic window either to provide a pill, switching on a pathway which can deal with the noise stresses or indeed perhaps other chemical stresses then that may provide some protection or conversely once the hearing damage is established. What the field suggests now is that those pathways that lead to irreversible damage principally to the hair cells and the auditory neurons are activated over a period of hours or potentially days after the initial insult. These are the challenges I think for the hearing science to identify which pathways within particular cells in the hearing organ are key to preserving hearing function.